Why put your packaging needs into game-data-packager?

1. game-data-packager (gdp) runs as a normal user. postinst scripts run
   as root. This reduces the impact of a security flaw in the scripts.
2. the .deb packages that gdp generates can be archived by the end user
   and installed on multiple machines, or re-installed (rather than
   regenerated) after a clean install.
3. postinst scripts need to run in a non-interactive context which makes
   prompting the user difficult (or requires you to abuse debconf). gdp
   scripts can expect an interactive terminal for prompting.
4. download requirements in gdp can be much more complex than you may be
   comfortable implementing in a postinst script: multiple mirrors, mirror
   selection, parallel downloads, retries, user-supplied local copies of
   files to download, proxy specification, etc.
5. common needs between packages (e.g. download logic, patch logic, etc.)
   can be in shared routines.
6. other non-live stuff can be shared (icons, package descriptions, etc.)
   in some cases (e.g. variants of the same game such as the Doom family)
7. the .deb files that gdp generates have a correct Installed-Size.